Wings which redirect air to waggle
sideways could cut airline fuel bills by 20% according to research
funded by the Engineering and Physical Sciences Research Council
(EPSRC) and Airbus in the UK.

The new approach, which promises to
dramatically reduce mid-flight drag, uses tiny air powered jets
which redirect the air, making it flow sideways back and forth
over the wing.

The jets work by the Helmholtz
resonance principle --- when air is forced into a cavity the
pressure increases, which forces air out and sucks it back in
again, causing an oscillation – the same phenomenon that happen
when blowing over a bottle.

Dr Duncan Lockerby, from the
University of Warwick, who is leading the project, said: “This has
come as a bit of a surprise to all of us in the aerodynamics
community. It was discovered, essentially, by waggling a piece of
wing from side to side in a wind tunnel.”

“The truth is we’re not exactly sure
why this technology reduces drag but with the pressure of climate
change we can’t afford to wait around to find out. So we are
pushing ahead with prototypes and have a separate three year
project to look more carefully at the physics behind it.”

Simon Crook, EPSRC Senior manager for
aerospace & defence, said: "This could help drastically reduce
the environmental cost of flying. Research like this highlights
the way UK scientists and engineers continue to make significant
contributions to our lives."

Notes for editors:

The UK aviation industry has announced
targets to reduce emissions per passenger km by 50% by 2020.

Part of these savings will be made
from lighter aircraft plus improvements in engines and fuel
efficiencies but drag friction is also a major factor in fuel
consumption during flights.

Engineers have known for some time
that tiny ridges known as ‘riblets’ - like those found on sharks
bodies - can reduce skin-friction drag, (a major portion of
mid-flight drag), by around 5%. But
the new micro-jet system being developed by Dr Lockerby and his
colleagues could reduce skin friction drag by up to 40%,

The research, being carried out with
scientists at Cardiff, Imperial, Sheffield, and Queen's University
Belfast, is still at concept stage although it is hoped the new
wings could be ready for trials as early as 2012.

If successful this technology could
also have a major impact on the aerodynamic design and fuel
consumptions of cars, boats and trains.

The Engineering and Physical Sciences Research Council (EPSRC) is
the UK’s main agency for funding research in engineering and the
physical sciences. The EPSRC invests more than £740 million a year
in research and postgraduate training, to help the nation handle
the next generation of technological change.

Airbus's aim to reduce fuel burn per passenger km by at
least 50% by 2020 will be difficult to achieve without a 30 to
50% reduction in skin-friction drag / the drag arising from the
friction generated on the aircraft's surface by the direct
action of the air flow. We propose, therefore, to investigate
novel, practical, effective flow-control techniques for
achieving this aim.Skin-friction drag in turbulent boundary layers is
governed by the flow physics very close to the surface in a
region of the flow field known as the viscous sublayer. The
generation of wall friction is also known to be quasi-cyclic. An
essential characteristic of this cycle and the near-wall flow
physics are streaks of low- and high-speed flow and their strong
interaction with wave-like disturbances. The resulting evolution
of the streaks and their explosive growth are intimately
connected with the generation of wall friction and thereby drag.Most researchers focus on these sublayer streaks
because they are very closest to the wall and amenable to
wall-based actuation and sensing. We estimate, however, that
there are O(109) sublayer streaks over the fuselage of an Airbus
A340-300 at any instant during cruise. Others have made similar
estimates. This enormous number makes it utterly impractical to
implement an active control strategy targeting streaks
individually. But disrupting the cycle in a global untargeted
way is feasible. Riblets (minute peaks and troughs running in
the flow direction with crossflow spacing of about 1/3 of a
human hair width) do this by disrupting streak growth, in effect
by regularizing and partially stabilizing them. But conventional
riblets only deliver less than 1.5% drag reduction in flight
tests, although 6% is achieved in idealized laboratory
experiments. Unless this poor performance can be greatly
improved, riblets are of little practical interest. Spanwise
oscillations have been studied recently and shown to be much
more effective than riblets at reducing skin-friction drag.
Again these appear to work by forcing the streaks into more
stable orientations. But this technique requires substantial
power input. Given the cyclic process described above, another
option is to disrupt the interaction of the waves and streaks
with randomized perturbations. This was tried by Sirovich et al.
who obtained 12% drag reduction in experimental flow studies
with randomized surface roughness elements. This approach has
not really been further investigated, although disrupting the
wave-streak interaction with randomized perturbations is likely
to be much more effective than riblets.We propose to investigate: (i) the use of randomized
distributions of small-scale Helmholtz resonators that create
strong microjets without any power input; thus are likely to be
more effective than roughness elements or riblets; (ii)
conventional riblets localize the streaks, thus combining them
with resonators could be much more effective than riblets alone;
(iii) improving effectiveness with unconventional riblets; e.g.,
wavy riblets mimicking spanwise oscillations and other 3D
patterns. Our study will be based on our simplified theoretical
model of the sublayer streaks which can be used at flight
Reynolds number. Helmholtz resonators hold great promise as passive
control devices because: (i) the control disturbance produced is
proportionately much greater than for roughness elements,
including riblets; (ii) they require no power input; and (iii)
consisting simply of a cavity with a necked exit orifice, they
are straightforward to manufacture at MEMS (micro) scale.Final Report
Summary

Airbus is seeking to develop
technologies that will enable the ACARE VISION 2020 targets to
be met, allowing sustained air travel growth whilst having zero
additional environmental impact. Achieving this requires a 50%
reduction in both fuel consumption and CO2 emissions. To meet
the target of reducing fuel burn per passenger km by at least
50% by 2020 will be difficult without a 30 to 50% reduction in
skin-friction drag.At Warwick and Cardiff, we have
developed efficient numerical methods to explore a range of
novel, but practical, passive control strategies (i.e. those
requiring no power input) capable of reducing skin-friction drag
on passenger jet aircraft. Importantly, these simulation tools
are capable of modelling basic configurations at flight-scale
Reynolds numbers (far beyond the reach of conventional
computational methods). Using these techniques, the
characteristics of critical near-wall flow structures
('streaks') at cruise conditions have been estimated, and a very
substantial drag reduction (~40%) has been demonstrated using
spanwise flow oscillations at the flight scale. Passive means
for creating spanwise oscillations have been explored, and the
potential of using Helmholtz resonators (small plenum chambers,
sunk beneath the aircraft surface) to generate oscillating jets
has been identified. Low-speed Direct Numerical Simulations
(coupled with an actuator model) have shown that, driven by
boundary-layer pressure fluctuations, a strong and coherent jet
flow is generated through the orifice of the resonator - such a
response, in ensemble, could be harnessed to create spanwise
forcing without the need for electrical input. The key
achievements, which directly shape and support the follow-on
EPSRC/Airbus project "Scalable Wirelessly Interconnected
Flow-Control Technologies (SWIFT)" EP/G038686/1, are: > Development of a reduced-order
fluid dynamics model for basic simulations at flight scale. > Demonstration of major drag
reduction at cruise speeds using spanwise forcing. > Discovery of a promising
passive strategy for the generation of spanwise forcing (using
Helmholtz resonators). http://www.springerlink.com/content/u966rk46202262r9/Flow,
Turbulence
and Combustion, Volume 78, Numbers 3-4 / June, 2007,
Pages 205-222

Abstract -- A
theoretical analysis is described that determines the conditions
for Helmholtz resonance for a popular class of self-contained
microjet actuator used in both synthetic- and pressure-jump
(pulse-jet) mode. It was previously shown that the conditions for
Helmholtz resonance are identical to those for optimizing actuator
performance for maximum mass flux. The methodology is described
for numerical-simulation studies on how Helmholtz resonance
affects the interaction of active and nominally inactive micro-jet
actuators with a laminar boundary layer. Two sets of numerical
simulations were carried out. The first set models the interaction
of an active actuator with the boundary layer. These simulations
confirm that our criterion for Helmholtz resonance is broadly
correct. When it is satisfied we find that the actuator cannot be
treated as a predetermined wall boundary condition because the
interaction with the boundary layer changes the pressure
difference across the exit orifice thereby affecting the outflow
from the actuator. We further show that strong inflow cannot be
avoided even when the actuator is used in pressure-jump mode. In
the second set of simulations two-dimensional Tollmien–Schlichting
waves, with frequency comparable with, but not particularly close
to, the Helmholtz resonant frequency, are incident on a nominally
inactive micro-jet actuator. The simulations show that under these
circumstances the actuators act as strong sources of 3D
Tollmien–Schlichting waves. It is surmised that in the real-life
aeronautical applications with turbulent boundary layers broadband
disturbances of the pressure field, including acoustic waves,
would cause nominally inactive actuators, possibly including
pulsed jets, to act as strong disturbance sources. Should this be
true it would probably be disastrous for engineering applications
of such massless microjet actuators for flow control.

C. Mares & D.A. LOCKERBY 2006, Developing professional skills
through group design projects and participation in student
competitions. International Conference on Innovation, Good
Practice and Research in Engineering Education, Liverpool, 2006